When a universal joint of this type operates, there are variations in temperature which lead in particular to an increase in the pressure inside the joint and a deformation of the tight casing; such deformation, which is increased by the effect of the centrifugal force on the grease in the joint, tends to cause rapid deterioration of the tight casing.
To overcome this drawback, joints have been made in which the space inside the casing was connected to the atmosphere by a device interposed between the driving shaft and the casing and provided to this end with an inner groove.
For example, French Pat. No. 2 414 145 describes a universal joint in which the device for communicating with the atmosphere is constituted by a split elastic ring which is disposed between the tight casing and the driving shaft and is shaped so as to leave an air passage along the driving shaft when the sleeve of the casing is clamped on this shaft.
However, this device for communicating with the atmosphere has a number of drawbacks. When the driving shaft does not present a groove where the bellows is held, the elastic ring which has a relatively low coefficient of friction, less than that of the casing, may slide on the driving shaft. Furthermore, in the case of sliding universal joints in which the inner volume of the joint may vary rapidly, the grease contained in the casing may emerge through the groove of the ring.